1. Field of the Invention
The present invention relates to a technical field regarding an optical pickup and a disk drive apparatus and particularly relates to a technical field for improving quality of a tracking error signal and suppressing effects of eccentricity of a disk-like recording medium or the like.
2. Description of the Related Art
Disk drive apparatuses for recording or reproducing an information signal on and from a disk-like recording medium are known. Such a disk drive apparatus includes an optical pickup movable in a radial direction of the disk-like recording medium mounted on a disk table for recording or reproducing the information signal by irradiating a laser beam through an objective lens on the disk-like recording medium.
In the optical pickup, the focusing adjustment is carried out by shifting an objective lens in far and near directions (focusing direction) with respect to a recording surface of the disk-like recording medium in response to the detection of a focusing error signal. Further, a tracking error signal is detected to perform tracking adjustment by shifting the objective lens in a substantially radial direction of the disk-like recording medium (tracking direction).
A push-pull method is known as a method of detecting the tracking error signal. However, there is a problem in this method that a large dc variation (DC offset) signal tends to occur when the objective lens is displaced in the tracking direction.
Accordingly, as a method of detecting the tracking error signal, a differential push-pull method capable of reducing the DC offset signal is widely used (for example, see Japanese Patent Application Publication 61-94246).
In the differential push-pull method, a laser beam is split into a main beam and a pair of sub-beams by a diffraction element, and as shown in FIG. 1, each of the sub-spots S, S, formed on a recording surface of a disk-like recording medium, is located between adjacent tracks T, T in such a way that the distances in the radial direction of the disk between the sub-spots S, S and the spot caused by the main beam (main spot) M are set to a half of a track pitch P, respectively.
As mentioned above, in the differential push-pull method, by making the distances in the radial direction of the disk between each of the sub-spots S, S and a spot caused by the main beam (main spot) M half of a track pitch P, the DC offset signal is cancelled by inverting the phase of the tracking error signal detected from the main beam and the phase of the tracking error signal detected from the sub-beams (see FIG. 2).